Energy evolution and constitutive modeling of thermo-mechanical behavior in sandstone under real-time high temperature conditions

With the growing prevalence of underground engineering in high-temperature settings, understanding the thermo-mechanical behavior of rock masses has become essential for ensuring engineering rock mass stability. The focus of this study is the mechanical and energy characteristics of sandstone under real-time thermo-mechanical coupling with small temperature gradients (20–150 °C) and establishes a corresponding statistical damage constitutive model. Experimental results reveal a distinct temperature-dependent “grouping effect”: minor strength degradation occurs at 20–60 °C, whereas significant weakening is observed at 90–150 °C, attributed to a transition from evaporation-induced pore pressure to differential thermal expansion and microcracking. A novel axial stress ratio approach clearly delineates three energy evolution stages and a progressive decline in energy storage capacity with temperature, with all energy components exhibiting consistent grouping behavior and linear increase with confining pressure. The proposed constitutive model accurately captures stress-strain responses under thermo-mechanical coupling and reflects the material's brittle-ductile transition. It is concluded that sandstone behavior under coupled conditions is governed by the competition between thermal degradation and confining pressure strengthening. These findings provide theoretical and practical insights for rock engineering in thermal environments.